Kinnel (2004) described the transportation of soil particles under raindrop erosion in following four processes: 1. Raindrop Detachment with Transport by Raindrop Splash (RD-ST) Raindrop Detachment with Transport by Raindrop Induced Flow Transport (RD-RIFT) 3. Raindrop Detachment with Transport by Flow (RD-FT) 4. Flow Detachment with Transport by Raindrop (FD-FT).
These processes are described as under:
1. Raindrop Detachment with Transport by Raindrop Splash (RD-ST):
This process is expressed by the notation RD-ST, in which RD stands for raindrop detachment and ST for splash transport. In general, the splash erosion is referred by this process (RD-ST). In this process, the detachment and transportation of soil particles is done by falling raindrops.
During rainfall the raindrop falls over the ground surface with some KE. A part of this KE is consumed to overcome the bonding force between the soil particles; and a part of the same is utilized in transporting the detached soil particles away from the point of detachment.
The detachment of soil particles is only happened, when KE of falling raindrop is more than the bonding force; otherwise there is no. The rain splash transports the soil particles radially from the detachment point. This mode of transportation takes place at the time of onset of rainfall, when there is little or no flow of water over the soil surface. In this process, the transportation of soil particle is not very significant; or in other words, it is being inefficient. The transportation distance depends on the soil surface slope.
If slope is flat or no slope, then soil particles splashed by an individual raindrop from a given point, are replaced with the soil particles splashed by other raindrops in surrounding area. On the other hand, if surface slope is there, then detached soil materials are transported to some distance, down slope.
In this case, the splashed particles are not replaced by the other splashed particles. The transportation distance is more towards down slope, as compared to the upslope. Transportation distance depends on the slope steepness. If slope is more, then there is greater transportation distance; and vice-versa.
For occurrence of soil particle detachment due to splash erosion the KE of falling raindrop must be greater than the critical value, is presented by the following relationship–
When e ≥ ec
Dr = Kd (e – ec)b …(3.50)
and when Dr = 0 …(3.51)
In which Dr is the weight of soil particle detachment; Kd is the soil detachability coefficient; b is empirical constant; e is the KE of falling raindrop and ec is the critical KE. The value of critical KE depends on the soil materials. For example – in the case of loose soils the particles are easily detached by the raindrop, causing the value of ec to be relatively less. On the other hand, in heavy soils it is more. The cohesion and inter particle friction force and soil moisture content also affect the critical KE of the rainfall. The value of ec is being more due to surface crusting (Fig. 3.11).
The equation (3.50) can be used for predicting the splash erosion from larger area by introducing transportation efficiency, which depends on the slope gradient of topsoil surface.
The modified equation is given as below:
in which, S is the amount of transported materials in a given time; KsD is the soil credibility associated to splash erosion; Ex is the effective rainfall energy applied to the soil surface to detach the soil particles and f(Sg) is the function that depends on the ground slope gradient. The value of Ex is given by Foley and Silburn (2002) as under –
in which, ei is the KE of ith raindrop.
The value of KsD is not equal to single value of Kd because the already detached materials, which have been deposited on the soil surface, provide protection to the soil surface against falling raindrops. However, in effects the two extreme value of Kd is taken into consideration; in which first is the Kdm is the soil credibility when there is no predetached soil particles on the soil surface; and raindrops detach the soil particles only from the soil matrix.
And, second is the Kd PD, i.e., when there is sufficient depth of predetached soil materials on the soil surface. In second condition the raindrop first gets penetrate into the soil layer and then detaches the particles from there; and soil particles get splash only from the deposited soil layer. In case of deposition of predetached materials, there is created a kind of protection to the top soil surface. If this is taken as the degree of protection Hra then the value of KsD is given as under–
The value of Hra varies with space and time, both. For large and flat surface its value could be 1, because of negligible transport capacity of RD-ST. But if there is few gradient in the soil surface, then the value of KsD becomes equal to the KD PD under steady state condition, despite the value of Hra as small.
Terry (1998) has identified following three stages of raindrop impact when soil surface is not under water layer:
i. Collision and deformation of falling raindrop on the soil surface.
ii. Rupture and collapse of raindrops into thin disk of water spraying radically outwards from the impacting point.
iii. Splashing or jetting of ejected droplets in parabolic trajectories away from the original drop falling point.
Amongst above three stages, the stage 1 and 2 are involved in soil detachment and in modification of characteristics of soil surface causing detachment. The actions of these two stages depend very much on the physio-chemical properties of soil.
Initially the wetted soil surface may produce structural breakdown because of internal/external forces involved with the entry of water into the aggregates. Also, in clay soils there is the possibility of hydration of clay particles, which leads to dispersion of soil particles. The falling raindrops can cause break down of soil aggregates and soil compaction, which could result into reduction in soil roughness.
The stage-3 causes transportation of detached soil particles. And because of change in roughness of soil surface, there is possibility of change in splash trajectories and thereby the splashing distance, also. The splash trajectory is significantly affected by the presence of water layer on the soil surface.
Allen (1987) mentioned that the splash trajectories can vary between 50 and 70° with thin water layer; and it could be more with increase in depth of water layer on the soil surface. This phenomena causes reduction in splashing of detached materials. Also, the detachment gets affect due to presence of water depth over the soil surface, because of absorption of KE of falling raindrop by the water layer.
The effect of slope gradient on net transport of soil materials on down slope under RD-ST process has been reported to be linear in few cases, while in few cases it is non-linear.
The form of relationship is given as under:
Qns = a Eb Gc …(3.54)
in which, Qns is the amount of splashed materials, down slope; E is the total KE of rainfall; G is the slope gradient (%); and a, b and c are empirical constants.
Quansah (1981) determined the value of c as below:
i. Sand and sandy loam soil – 0.7 to 1.0
ii. Sill loam, silty clay – 1.1 to 1.4 Clay loam and clay soils.
2. Raindrop Detachment with Transport by Raindrop Induced Flow Transport (RD-RIFT):
In this process the detachment of soil particle takes place by means of falling raindrops; and transportation of detached particles is by the raindrop induced water flow. In this case, due to continuation in rainfall, there develops water flow over the soil surface. And at the same time, the falling raindrops penetrate through the flow, and detach the particles from the soil surface. The detached particles get splashed because of breaking of raindrops. The splashed soil particles fall onto the flowing water, which are carried away to another place, downstream.
This process is continued due to subsequent falling raindrop. Since, the transportation process is accomplished due to raindrop impact and flowing water, both, therefore, this process is also referred as Raindrop Induced Flow Transport (RIFT). In this process, the coarser soil particles are transported in the form of rolling action.
As compared to ST process, this is more efficient process regarding particles transportation. From inter-rill areas, major portion of soil materials are transported by this process. This process is being transport limiting. Figure 3.12 presents the view of RIFT process.
This process normally takes place when flow does not have sufficient energy to move the materials, unless the rain drops impact the flow and under lying materials gets disturbed. Also, when stream power is greater than the critical stream power to move the pre-detached particles sitting on the bed then this process comes in action. In this condition, the falling raindrops lift the particles up from the bed into the moving water flow. These soil particles get fall in downstream direction when they return to the bed.
Fig. 3.12 presents the view of particle lifting due to drop impacting in non-turbulent flow condition. Simultaneously, the falling particles on the flow bed move horizontally by flowing water. The horizontal distance moved by the particle depends on particle size and density, drop size, flow depth and turbulence, mainly. The impacting drop controls the moving of particles horizontally by creating turbulence in water flow.
3. Raindrop Detachment with Transport by Flow (RD-FT):
In this process, the raindrop performs the detachment of soil particle; and transport of detached particles is done by a thin sheet of water flow. In most of the cases, it has been observed that, a thin flow can carry the soil material easily but cannot scour the particles from the soil surface. But if beating action is there due to falling raindrop, then particles scouring could get possible. In comparison to RD-RIFT, this process is more efficient to transport the soil materials.
In general, the RD-RIFT and RD-FT processes occur simultaneously in the same flow of water during rainfall occurrence. But difference is that, the coarser soil particles are transported under RD-RIFT process, while fine particles are under RD-FT process. Also, the RIFT process involves water flow.
In brief, this process is described under following points:
i. The particles detached and lifted from the soil matrix remain suspended for sufficient duration to get remove from the eroded area without returning to the bed.
ii. The detached and lifted soil particles from soil matrix return to the bed, but the flowing water stream without the aid of raindrop impact, carries them away.
In both of the above cases, the travel distance of soil particles after their detachment is greater than the distance from the point of detachment and their discharge. This is because of the reason that the particles do not require any aid from the raindrop impact. Normally, in this process the particles are moving with greater velocity to that of the RIFT process. The RIFT process transports the coarser soil particles, while very fine soil particles are by the FT process on short slopes, as observed in inter-rill areas.
The FT process becomes very dominating relatively on short slope with steep gradient. Normally, the RIFT and FT processes are found in action, simultaneously in the same flow. In same field the particles (fine) moving under FT process get reach to the end of eroding area earlier than the particles (coarser) moving under RIFT process. Due to this reason, in a given volume of sediment load the proportion of fine particles is greater than the coarser particles.
However, with the time advancement the share of coarser particles gets increase; and in this way the sediment load gets coarsened. The amount of materials detached by raindrop impact that moves by FT process depends on the degree of protection created by the pre-detached materials against detachment. If MRD-FTM, amount of soil mass detached by rain drop impact and moved by FT process when there is no pre-detached soil particles, then MRD-FTM is given by the following relationship–
If the value of HRb is equal to 1.0, then particles moving will not get detached. The speed of movement of particles under FT process is the same to the flow velocity, provided that they are moving in the form of suspended load or at lower velocity as in the case of saltation. The amount of sediment discharge by RD-FT process under steady state condition is given by the following relationship –
In which X is the length of flow over the surface where HRh < 1.0 and HRbX is the effective degree of protection provided by pre detached particles over the surface. The value of X varies with the degree of protection made by the detached particles moving under RIFT process, sitting on the bed. The protective effect of detached particles depends on the flow velocity. If the protective effect is nil then sediment discharge due to RD-FT does not get change with the variation in flow velocity. At this situation, the X becomes, equal to the length of eroding area; and the value of (1 -HRbX) approaches 1.0.
The rills are likely to get develop when moving water flow detaches the particles from soil surface; and transportation of detached materials is carried out along the flow. It is generally found that the detachment by flow gets reduce by the transported sediment under FT process. In WEPP model the rate of detachment by flow is given by the following relationship –
in which, DF is the rate of soil detachment due to flow; KF is the soil credibility associated to the rill erosion; τ is the shear stress of flow; τ0 is the soil dependent critical shear stress; qs is the sediment discharge rate; and TcF is the transport capacity of flow in terms of sediment discharge. In WEPP model, the q, refers to the amount of materials coming from inter-rill flows and raindrop splash, both. Irrespective of any detachment by the flow from rills, the development of rills in the area enhances the level of transport of soil materials due to raindrop detachment.
4. Flow Detachment with Transport by Raindrop (FD-FT):
This is the last phase of transportation of soil materials under raindrop erosion. In this process, the detachment and transport of soil particles is done by the flowing water. Flowing water detaches the soil materials, only when critical stream flow power exceeds the soil strength (credibility). The stream flow power depends on the flow rate and gradient of soil surface. The rill erosion takes place only due to FD process.